[MessageLoop] Fix flaky death-on-destruction test
gtest complains about the multi-threadedness of these tests not being
supported in POSIX death tests. The multi-threadedness is caused by
the WITH_TASK_SCHEDULER test variant which is unnecessary to test this.
Hence, detaching the test from the fixture and testing independently
will fix the flake I think.
R=danakj@chromium.org
Bug: 860733
Change-Id: I0972e3c65614145825b8b40d5bb2fa9ccea9108b
Reviewed-on: https://chromium-review.googlesource.com/1134346
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574558}